This invention relates to electro-optical devices whose electromagnetic radiation absorption characteristics can be selectively altered by influence of a suitably controlled electric field. More particularly, this invention is directed to a sandwich type cell in which two layers of electrochromic material are separated by a self-supporting ion conducting medium.
In commonly assigned, copending U.S. applications, Ser. No. 41,153 .Iadd.now abandoned, .Iaddend.Ser. No. 41,154 .Iadd.now abandoned .Iaddend.and Ser. No. 41,155 .Iadd.now U.S. Pat. 3,708,220, .Iaddend.all filed May 25, 1970, and U.S. Pat. Nos. 3,521,941 and 3,578,843.[.; Ser. No. 41,153, abandoned and refiled as Ser. No. 211,857, Dec. 23, 1971, abandoned and refiled as Ser. No. 361,760, May 18, 1973, now copending; Ser. No. 41,154, abandoned and refiled, now pending; Ser. No. 41,155, now U.S. Pat. No. 3,708,220;.]..Iadd., .Iaddend.there are described electro-optical devices exhibiting a phenomenon known as persistent electrochromism wherein electromagnetic radiation absorption characteristic of a persistent electrochromic (EC) material is altered under the influence of an electric field. Such devices were employed in sandwich arrangement between two electrodes. Coloration was induced by charging the electrochromic film negative with respect to the counter-electrode, employing an external potential. The counter-electrode can be the same as the persistent electrochromic material or different.
By reversing the original polarity of the field or by applying a new field, it was also possible to cancel, erase or bleach the visible coloration.
These steps of color induction and erasure are defined as cycling.
The devices described in the prior applications are effective to change their electromagnetic radiation transmitting properties under the influence of an electric field, and have extremely good visibility over a wide range of lighting conditions, including high ambient light. However, these EC displays normally feature a light color background which consists of a thin layer of pigment mixed with a liquid electrolyte to form an ion conducting layer. This layer serves to hide the black counter-electrode and to provide good contrast with the blue-black EC film. It has been found that EC displays when stored for long periods of time, especially when stored on edge, develop a problem of separation of the pigment background. This appears as a black crack in the light background due to the carbon counter-electrode showing through. Efforts to cope with this problem by providing thickened, gel-like electrolyte pastes resulted in slowing the switching speed as the thickened electrolyte reduced the mobility of ions between the electrodes.
It is, therefore, an object of this invention to provide an ion-conducting medium having a color pigment incorporated which will retain a homogeneous consistency under varying physical conditions, over a long period of time.
This and other objects of the invention will become apparent as the description thereof proceeds.